mtouch Inductive Touch User s Guide

Transcription

1 mtouch Inductive Touch User s Guide 2009 Microchip Technology Inc. DS41407A

2 Note the following details of the code protection feature on Microchip devices: Microchip products meet the specification contained in their particular Microchip Data Sheet. Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions. There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip s Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property. Microchip is willing to work with the customer who is concerned about the integrity of their code. Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as unbreakable. Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our products. Attempts to break Microchip s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act. Information contained in this publication regarding device applications and the like is provided only for your convenience and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. MICROCHIP MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED, WRITTEN OR ORAL, STATUTORY OR OTHERWISE, RELATED TO THE INFORMATION, INCLUDING BUT NOT LIMITED TO ITS CONDITION, QUALITY, PERFORMANCE, MERCHANTABILITY OR FITNESS FOR PURPOSE. Microchip disclaims all liability arising from this information and its use. Use of Microchip devices in life support and/or safety applications is entirely at the buyer s risk, and the buyer agrees to defend, indemnify and hold harmless Microchip from any and all damages, claims, suits, or expenses resulting from such use. No licenses are conveyed, implicitly or otherwise, under any Microchip intellectual property rights. Trademarks The Microchip name and logo, the Microchip logo, dspic, KEELOQ, KEELOQ logo, MPLAB, PIC, PICmicro, PICSTART, rfpic and UNI/O are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. FilterLab, Hampshire, HI-TECH C, Linear Active Thermistor, MXDEV, MXLAB, SEEVAL and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A. Analog-for-the-Digital Age, Application Maestro, CodeGuard, dspicdem, dspicdem.net, dspicworks, dsspeak, ECAN, ECONOMONITOR, FanSense, HI-TIDE, In-Circuit Serial Programming, ICSP, Mindi, MiWi, MPASM, MPLAB Certified logo, MPLIB, MPLINK, mtouch, Octopus, Omniscient Code Generation, PICC, PICC-18, PICDEM, PICDEM.net, PICkit, PICtail, PIC 32 logo, REAL ICE, rflab, Select Mode, Total Endurance, TSHARC, UniWinDriver, WiperLock and ZENA are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. SQTP is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. 2009, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. Microchip received ISO/TS-16949:2002 certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona; Gresham, Oregon and design centers in California and India. The Company s quality system processes and procedures are for its PIC MCUs and dspic DSCs, KEELOQ code hopping devices, Serial EEPROMs, microperipherals, nonvolatile memory and analog products. In addition, Microchip s quality system for the design and manufacture of development systems is ISO 9001:2000 certified. DS41407A-page Microchip Technology Inc.

3 INDUCTIVE TOUCH DEMO BOARD USER S GUIDE Table of Contents Chapter 1. Introduction 1.1 Introduction Development Kit Contents Inductive Touch Demonstration Board Sample Code Chapter 2. Getting Started 2.1 Inductive Touch Demo Board as a Stand-Alone Board Preprogrammed Device Inductive Touch Demo Board Used with an In-Circuit Debugger Inductive Touch Demo Board Connection to the mtouch GUI Chapter 3. Theory of Operation 3.1 Sensor Coil Impedance Measurement System Polling Software Appendix A. System Detail A.1 Sensor Design A.1.1 Fascia A.1.2 Targets A.1.3 Spacer A.1.4 Sensor Coils A.2 Hardware Design A.3 Power Supply A.4 Software Design A.5 Board Layout and Schematic Microchip Technology Inc. DS41407A-page 3

4 Inductive Touch Demo Board User s Guide NOTES: DS41407A-page Microchip Technology Inc.

5 INDUCTIVE TOUCH DEMO BOARD USER S GUIDE Preface NOTICE TO CUSTOMERS All documentation becomes dated, and this manual is no exception. Microchip tools and documentation are constantly evolving to meet customer needs, so some actual dialogs and/or tool descriptions may differ from those in this document. Please refer to our web site ( to obtain the latest documentation available. Documents are identified with a DS number. This number is located on the bottom of each page, in front of the page number. The numbering convention for the DS number is DSXXXXXA, where XXXXX is the document number and A is the revision level of the document. For the most up-to-date information on development tools, see the MPLAB IDE on-line help. Select the Help menu, and then Topics to open a list of available on-line help files. INTRODUCTION DOCUMENT LAYOUT This chapter contains general information that will be useful to know before using the Inductive Touch Demo Board. Items discussed in this chapter include: Document Layout Conventions Used in this Guide Warranty Registration Recommended Reading The Microchip Web Site Development Systems Customer Change Notification Service Customer Support Document Revision History This document describes how to use the Inductive Touch Demo Board as a development tool to emulate and debug firmware on a target board. The manual layout is as follows: Chapter 1. Introduction Introduces the Inductive Touch Demo Board and provides a brief description of the system. Chapter 2. Getting Started Goes through a basic step-by-step process for getting your Inductive Touch Demo Board up and running as a stand-alone board or with the mtouch GUI. Chapter 3. Theory of Operation Provides an overview of how the Inductive Touch system operates. Appendix A. System Detail Describes in detail the operation of the Demo Board hardware and software Microchip Technology Inc. DS41407A-page 5

6 Inductive Touch Demo Board User s Guide CONVENTIONS USED IN THIS GUIDE This manual uses the following documentation conventions: DOCUMENTATION CONVENTIONS Description Represents Examples Arial font: Italic characters Referenced books MPLAB IDE User s Guide Emphasized text...is the only compiler... Initial caps A window the Output window A dialog the Settings dialog A menu selection select Enable Programmer Quotes A field name in a window or Save project before build dialog Underlined, italic text with A menu path File>Save right angle bracket Bold characters A dialog button Click OK A tab Click the Power tab N Rnnnn A number in verilog format, 4 b0010, 2 hf1 where N is the total number of digits, R is the radix and n is a digit. Text in angle brackets < > A key on the keyboard Press <Enter>, <F1> Courier New font: Plain Courier New Sample source code #define START Filenames autoexec.bat File paths c:\mcc18\h Keywords _asm, _endasm, static Command-line options -Opa+, -Opa- Bit values 0, 1 Constants 0xFF, A Italic Courier New A variable argument file.o, where file can be any valid filename Square brackets [ ] Optional arguments mcc18 [options] file [options] Curly brackets and pipe Choice of mutually exclusive errorlevel {0 1} character: { } arguments; an OR selection Ellipses... Replaces repeated text var_name [, var_name...] Represents code supplied by user void main (void) {... } DS41407A-page Microchip Technology Inc.

7 Preface WARRANTY REGISTRATION RECOMMENDED READING Please complete the enclosed Warranty Registration Card and mail it promptly. Sending in the Warranty Registration Card entitles users to receive new product updates. Interim software releases are available at the Microchip web site. This user s guide describes how to use Inductive Touch Demo Board. Other useful documents are listed below. The following Microchip documents are available and recommended as supplemental reference resources. Readme for Inductive Touch Demo Board For the latest information on using Inductive Touch Demo Board, read the Readme for Inductive Touch Demo Board.txt file (an ASCII text file) in the Readmes subdirectory of the MPLAB IDE installation directory. The Readme file contains update information and known issues that may not be included in this user s guide. Readme Files For the latest information on using other tools, read the tool-specific Readme files in the Readmes subdirectory of the MPLAB IDE installation directory. The Readme files contain update information and known issues that may not be included in this user s guide. Reference Documents Reference documents may be obtained by contacting your nearest Microchip sales office (listed in the back of this document) or by downloading via the Microchip web site ( Recommended documents include: Individual data sheets and reference manuals: - PIC16F1936 Data Sheet (DS41364) - MPLAB IDE Simulator, Editor User s Guide (DS51025) - MPASM Assembler, MPLINK Object Linker, MPLIB Object Librarian User s Guide (DS33014) - MPLAB REAL ICE In-Circuit Emulator User s Guide (DS51616) - PICkit Serial Analyzer User s Guide (DS51647) - mtouch Sensing Solution User s Guide (DS41328) - PICkit 3 Programmer/Debugger User s Guide (DS51795) - MPLAB ICD 3 In-Circuit Debugger User s Guide (DS51766) 2009 Microchip Technology Inc. DS41407A-page 7

8 Inductive Touch Demo Board User s Guide THE MICROCHIP WEB SITE Microchip provides online support via our web site at This web site is used as a means to make files and information easily available to customers. Accessible by using your favorite Internet browser, the web site contains the following information: Product Support Data sheets and errata, application notes and sample programs, design resources, user s guides and hardware support documents, latest software releases and archived software General Technical Support Frequently Asked Questions (FAQs), technical support requests, online discussion groups, Microchip consultant program member listing Business of Microchip Product selector and ordering guides, latest Microchip press releases, listing of seminars and events, listings of Microchip sales offices, distributors and factory representatives DEVELOPMENT SYSTEMS CUSTOMER CHANGE NOTIFICATION SERVICE Microchip s customer notification service helps keep customers current on Microchip products. Subscribers will receive notification whenever there are changes, updates, revisions or errata related to a specified product family or development tool of interest. To register, access the Microchip web site at click on Customer Change Notification and follow the registration instructions. The Development Systems product group categories are: Compilers The latest information on Microchip C compilers and other language tools. These include the MPLAB C18 and MPLAB C30 C compilers; MPASM and MPLAB ASM30 assemblers; MPLINK and MPLAB LINK30 object linkers; and MPLIB and MPLAB LIB30 object librarians. Emulators The latest information on Microchip in-circuit emulators.this includes the PICkit 3, MPLAB ICD 3, and MPLAB REAL ICE. MPLAB IDE The latest information on Microchip MPLAB IDE, the Windows Integrated Development Environment for development systems tools. This list is focused on the MPLAB IDE, MPLAB SIM simulator, MPLAB IDE Project Manager and general editing and debugging features. DS41407A-page Microchip Technology Inc.

9 Preface CUSTOMER SUPPORT Users of Microchip products can receive assistance through several channels: Distributor or Representative Local Sales Office Field Application Engineer (FAE) Technical Support Customers should contact their distributor, representative or field application engineer (FAE) for support. Local sales offices are also available to help customers. A listing of sales offices and locations is included in the back of this document. Technical support is available through the web site at: DOCUMENT REVISION HISTORY Revision A (November 2009) Initial Release of this Document Microchip Technology Inc. DS41407A-page 9

10 Inductive Touch Demo Board User s Guide NOTES: DS41407A-page Microchip Technology Inc.

11 INDUCTIVE TOUCH DEMO BOARD USER S GUIDE Chapter 1. Introduction 1.1 INTRODUCTION Thank you for purchasing the mtouch Inductive Touch demonstration board from Microchip Technology Incorporated. The mtouch Inductive Touch is a simple board that demonstrates the capabilities of Microchip s Inductive Touch Technology. The Inductive Touch Demo Board can be used stand-alone or with an in-circuit debugger (for example, MPLAB ICE). The demo board can also be used with the PICkit Serial and the mtouch GUI. 1.2 DEVELOPMENT KIT CONTENTS The Inductive Touch Demo Board comes with the following: Inductive Touch demonstration board (Figure 1-1) CD-ROM that contains: - Sample code - Source files - Inductive Touch Demo Board User s Guide - Application Notes If you are missing any part of the kit, please contact your nearest Microchip sales office listed in the back of this publication for help. 1.3 INDUCTIVE TOUCH DEMONSTRATION BOARD The Inductive Touch demonstration board has the following hardware features: 4 touch sensor buttons 4 indicator LEDs Variable frequency tone generator 5V power supply ICSP header PICkit Serial header 2009 Microchip Technology Inc. DS41407A-page 11

12 Inductive Touch Demo Board User s Guide FIGURE 1-1: INDUCTIVE TOUCH HARDWARE 1.4 SAMPLE CODE The Inductive Touch Demo Board comes preprogrammed with working Inductive Touch software. The CD-ROM also includes the software hex file, and the source code for the software. This program may be used as-is in the demo board, with the REAL ICE In-Circuit Emulator (ICE) or with either the PICkit 3 or the ICD 3 In-Circuit Debugger. DS41407A-page Microchip Technology Inc.

13 INDUCTIVE TOUCH DEMO BOARD USER S GUIDE Chapter 2. Getting Started The Inductive Touch Demonstration Board may be used as a stand-alone board, with an in-circuit emulator (for example, MPLAB REAL ICE ) or with the PICkit Serial and the mtouch GUI. 2.1 INDUCTIVE TOUCH DEMO BOARD AS A STAND-ALONE BOARD PREPROGRAMMED DEVICE The Inductive Touch Demonstration Board may be demonstrated immediately by following the steps listed below: Apply power to the Inductive Touch Demo Board. For information on acceptable power sources, see Appendix A. Apply pressure to any of the 4 buttons, the associated LED should light, and a tone should sound. To reprogram the demo board microcontroller, the following will be necessary: Program source code user source code may be used to program the device, or if this previously has been done, the sample program may be restored from the file on the included CD-ROM. A compiler, such as the HITECH C compiler, or equivalent third party compiler, must be used to compile the source code into a hex file before it can be programmed into the device. Other compilers may be used. For a list of these PIC MCU compatible language tools, see the Microchip web site ( Once the sample program is in hex file format, a programmer can program the Flash device. A device programmer, such as PICkit 3, MPLAB ICD 3, or REAL ICE (programmer functionality available with MPLAB IDE v8.40 or greater) If the code protection bit(s) have not been programmed, the on-chip program memory can be read out for verification purposes. 2.2 INDUCTIVE TOUCH DEMO BOARD USED WITH AN IN-CIRCUIT DEBUGGER To use the Inductive Touch Demonstration Board with an In-Circuit Debugger (ICD), refer to the tool s user guide for instructions to learn how to: Power-up and configure the ICD Connect to target boards (such as in Figure 2-1) Program and debug the microcontroller on the demo board Microchip Technology Inc. DS41407A-page 13

14 Inductive Touch Demo Board User s Guide FIGURE 2-1: INDUCTIVE TOUCH DEMO BOARD CONNECTED TO PICkit 3 USING USB 2.3 INDUCTIVE TOUCH DEMO BOARD CONNECTION TO THE mtouch GUI The Inductive Touch Demonstration Board can be used with the mtouch GUI to demonstrate the operation of the system. Connect the Inductive Touch Demonstration Board to a PICkit Serial Analyzer as shown in Figure 2-2. FIGURE 2-2: CONNECTION TO A PICkit SERIAL ANALYZER Apply power to the Inductive Touch Demonstration Board. For information on acceptable power sources, see Appendix A. System Detail. DS41407A-page Microchip Technology Inc.

15 Getting Started In MPLAB (version 8.4 or later), select the mtouch Diagnostic Tool under the Tools menu (as shown in Figure 2-3). FIGURE 2-3: mtouch DIAGNOSTIC TOOL When the Diagnostic Tool is loaded, select the Settings tab to access the tool configuration menu (Figure 2-4). FIGURE 2-4: SETTINGS Under the Settings menu select the Custom option (Figure 2-5) Microchip Technology Inc. DS41407A-page 15

16 Inductive Touch Demo Board User s Guide FIGURE 2-5: SELECT BOARD DS41407A-page Microchip Technology Inc.

17 Getting Started Under the Communication tab, select the I2C option and enter an address of 42 (Figure 2-6). FIGURE 2-6: COMMUNICATION The Diagnostic Tool should now display the readings for the first 4 sensors on the display (Figure 2-7). FIGURE 2-7: FIRST 4 SENSORS READINGS For more information concerning the diagnostic tools, please check the Microchip web page at and the MPLAB help files Microchip Technology Inc. DS41407A-page 17

18 Inductive Touch Demo Board User s Guide NOTES: DS41407A-page Microchip Technology Inc.

19 INDUCTIVE TOUCH DEMO BOARD USER S GUIDE Chapter 3. Theory of Operation An Inductive Touch system uses the magnetic coupling between a solid metal target and an inductive sensing coil. If a user presses on the front panel, then the coupling between the target and sense coil will increase due to the minute shift in the target s position toward the sensor coil, as shown in Figure 3-1. The result is a change in the impedance of the sensing coil. FIGURE 3-1: CROSS SECTION OF AN INDUCTIVE TOUCH SENSOR 3.1 SENSOR COIL IMPEDANCE MEASUREMENT SYSTEM To sense this change in the inductive sensing coil, an impedance measurement system and a microcontroller are required. The impedance measurement system operates by exciting the sensor coil with a pulsed current. This produces a pulsed voltage across the coil that is proportional to both the current and the impedance of the coil. The impedance measurement system then converts the pulsed voltage into a DC voltage proportional to the amplitude of the pulsed voltage using a frequency mixer, low pass filter, and amplifier. The resulting DC value is converted to a binary number using an ADC. To assist in maintaining consistency in the readings, a reference coil has been added to remove variations in the readings due to long term variations in the current and due to temperature shifts. The reference coil is in series with the sensor coil, and is measured as part of the conversion process. The resulting value for the reference coil is divided into the sensor coil, resulting in a normalized value for the coil. By using the impedance ratio of the two coils, any variation in the impedances due to temperature or long term drive current variation fall out and we are left with a temperature and voltage compensated value. Figure 3-2 shows a block diagram of a typical impedance measurement system Microchip Technology Inc. DS41407A-page 19

20 Inductive Touch Demo Board User s Guide FIGURE 3-2: BLOCK DIAGRAM OF THE IMPEDANCE MEASUREMENT SYSTEM Driver PWM Sensor Coils Switching Filter Measurement Section 10-bit ADC Reference Coil 3.2 POLLING SOFTWARE The microcontroller periodically polls the various sensors by measuring impedance of the individual sensing coils. If the impedance of the sense coil has changed from the store average for each coil, then the microcontroller determines if the shift in impedance is sufficient to qualify as a user s press. Figure 3-3 is a flowchart of the system software. The polling process includes the measurement of the voltage across both coils, just the reference coil, and the virtual ground of the system. Subtracting the various values from one another produces voltages for both coil. Dividing the sensor coil voltage by the reference coil voltage, then gives a normalized value in which temperature and drive variations have been removed. The software then compares the normalized value against a running average to determine if the shift is sufficient for a valid press by the user. DS41407A-page Microchip Technology Inc.

21 Theory of Operation FIGURE 3-3: SYSTEM FLOWCHART Turn on Driver Select Reference Input Get ADC Select Sensor Input Get ADC Turn off Driver Get ADC Calculate Normalized Value Value < Threshold Yes No Average New Value Button Pressed Logic 2009 Microchip Technology Inc. DS41407A-page 21

22 Inductive Touch Demo Board User s Guide NOTES: DS41407A-page Microchip Technology Inc.

23 INDUCTIVE TOUCH DEMO BOARD USER S GUIDE Appendix A. System Detail A.1 SENSOR DESIGN An inductive touch system uses the magnetic coupling between a solid metal target and an inductive sensing coil. If a user presses on the front panel, then the coupling between the target and sense coil will change due to the minute shift in the target s position, as shown in Figure A-1. FIGURE A-1: TYPICAL SENSOR/TARGET SANDWICH FIGURE A-2: SENSOR/TARGET SANDWICH OF DEMO BOARD 2009 Microchip Technology Inc. DS41407A-page 23

24 Inductive Touch Demo Board User s Guide A.1.1 Fascia The fascia is typically the top layer of the sensor sandwich. It is the plastic panel or metal sheet that the user presses on and usually carries the labeling that identifies the button s location and function. The fascia can also be combined with the target if it is conductive, and can be sufficiently deformed by the user s press to cause the required minimum change in inductance (the Inductive Touch Demo Board uses a combined Fascia and Target). The material of the fascia can, in theory, be made from any material so long as it elastically deforms under the range of expected pressures that will be applied by the user. The fascia used in the Inductive Touch Demo Board is inches thick aluminum. With a button diameter of 0.8 inches, that means a press force of only 5½ oz. is required to generate the minimum deflection required for a user press. Other common fascia constructions include plastic injection moldings, plastic sheet, sheet or formed metal. Other materials can also be used by the user, provided they elastically deform under pressure. A.1.2 Targets The target is a passive, electrically conductive layer which is arranged to displace or deform along the measurement axis relative to the coil. If a separate fascia layer is used, then the target is mechanically bonded to the fascia, so it will deform with pressure on the fascia. While magnetically permeable materials such as ferrites can be used for the target, highly conductive materials are preferable, due to their good conductivity and lower cost. In a typical application, the target is made from a copper lamination, but it could also be made from any other highly conductive materials such as aluminum, gold, silver or steel. The target layer in the Inductive Touch Demo Board is aluminum, and approximately inches thick. Only a thin layer of conductive material is actually required for a target (typically <100 microns) and can be produced, for example, using 3 ounce copper cladding on a PCB substrate. The diameter target should be the same size and shape as the coil. A.1.3 Spacer Most but not all inductive touch constructions use a spacer layer to provide the separation between the target and the coils. Constructions which use an injection molded fascia do not typically require a spacer layer since the separation distance can be molded as an integral feature of the plastic. A spacer layer can also be built into the target layer through embossing (creating a raised bump in the target material). The embossed area of the target (immediately above the sensor coil) provides the necessary displacement that will allow the target to move. The thickness of the spacer should be approximately 1%-3% of the diameter of the coil. The spacer layer can be made from a wide range of materials but should be a mechanically stiff and electrically insulating material such as FR4, FR2, resin bonded paper, ABS or other plastic. The depth of embossing on the Inductive Touch Demo Board is approximately inch for a 0.8 inch diameter button, or approximately 1.2%. A.1.4 Sensor Coils The sensor coils are one or more inductors, preferably implemented as flat spiral coils, etched into the copper layer of a PCB, as shown in Figure A-3. DS41407A-page Microchip Technology Inc.

25 System Detail FIGURE A-3: SPIRAL INDUCTOR PCB PATTERN A.2 HARDWARE DESIGN The inductance of the coil is determined by the number of turns and the dimensions of the pattern etched into the PCB. Note that the calculated inductance of the coil is not particularly important as the target will change the impedance of the coil due to its proximity. The coils in the Inductive Touch Demo Board are approximately 0.8 inches in diameter, with 24 turns. This works out to a trace width of inch with inch spacing (0.008 width with spacing is also typically used). The uncovered inductance is, therefore, nominally 12 µh. To measure the impedance of a sensor coil, the measurement system must first excite the coil with a pulsed current. This produces a pulsed voltage across the coil that is proportional to both the current and the impedance of the coil. The impedance system then converts the pulsed voltage into a DC voltage proportional to the amplitude of the pulsed voltage. The resulting DC value is converted to a binary number using an ADC, and software in the system then decides if the shift in impedance is indicative of a user s touch. To assist in maintaining consistency in the readings, a reference coil has been added to remove variations in the readings due to long term variations in the current and due to temperature shifts. The reference coil is in series with the sensor coil, and is measured as part of the conversion process. The resulting value for the reference coil is divided into the sensor coil, resulting in a normalized value for the coil. By using the impedance ratio of the two coils, any variation in the impedances due to temperature or long term drive current variation fall out and we are left with a temperature and voltage compensated value. Figure A-4 shows a simplified schematic diagram of the Inductive Touch Demo Board s impedance measurement circuitry Microchip Technology Inc. DS41407A-page 25

26 Inductive Touch Demo Board User s Guide FIGURE A-4: SIMPLIFIED MEASUREMENT CIRCUITRY VDD PWM R1 C nF 1µF optional LS1 LS2 LS3.01µF.01µF Ref coil/ Btn coil 1K GPIO 1K 330PF 330PF Isolate/ Connect GPIO R4 R4 R5 C2 C2 R5 MCP K 10-bit ADC VDD GPIO LR MCP K 10K 1nF The first section of the design is the current driver, shown in Figure A-5. FIGURE A-5: CURRENT DRIVER CIRCUIT VDD 10 1nF 1µF PWM R1 C1 0 optional R1 and C1 form a low pass filter which rounds off the edges of the PWM square wave, to produce a more sine-like waveform for driving the coil. In the Inductive Touch Demo Board, R1 is 1K and C1 is 150pF, this puts the corner frequency at 1MHz which is the frequency of the PWM drive. The next section of the design is the switching circuitry for selecting individual sensor coils. Figure A-6 shows the section of the design. DS41407A-page Microchip Technology Inc.

27 System Detail FIGURE A-6: COIL SELECTION MULTIPLEXERS R1 C1 LS1 LS2 LS3 GPIO LR The analog multiplexer used in the Inductive Touch Demo Board is a MM74HC4052 from Fairchild Semiconductor. It was chosen for its extremely low on resistance of Ohms, which significantly increases the current drive to the coils. The multiplexers are also doubled, one on the drive side and one on the measurement side, to eliminate offset voltages that are generated across the on resistance of the drive side multiplexers. After the switching section is the detector circuit. Figure A-7 shows the schematic for this section. FIGURE A-7: SCHEMATIC OF THE DETECTOR SECTION Ref coil/ Btn coil GPIO Isolate/ Connect GPIO C2.01µF R4 R5 330PF R4 10-bit ADC.01µF 1K 1K 330PF R5 C2 10K VDD 10K 10K 1nF 2009 Microchip Technology Inc. DS41407A-page 27

28 Inductive Touch Demo Board User s Guide A.3 POWER SUPPLY The detector operates by switching the filter/amplifier circuit between an inverting and non-inverting configuration, in time with the drive waveform of the coils. This allows the detector to flip the negative side of the waveform, resulting in a positive signal similar to the output of a full wave rectifier. The capacitors in the feedback path of the amplifier/filter, average out the peaks and valleys to produce a DC level proportional to the AC signal s amplitude. The detector in the Inductive Touch Demo Board uses parallel combination of 100K and 330 pf to place the corner frequency at 5 khz. With a value of 10K for R4, this also gives a gain of 10 for the system. The multiplexer used for multiplexing the button/reference inputs, and the switching of the amplifier is an MM74HC4053, also from Fairchild. The second op amp at the bottom of the diagram provides a virtual ground at VDD/2 for the single supply circuit. In the demo board, both amps are part of an MCP6002 op amp device. The detector also uses two inputs from the microcontroller to control which waveforms are detected. The first selects between the button coil and the reference coil. The second, disables the multiplexer in the detector, to isolate the detector from the sensor coils. This is necessary to obtain a clean measurement of the detector VDD/2 virtual ground. All three measurements are required to convert the button coil impedance into a normalized value in the software. The power supply of the Inductive Touch Demo Board requires a 9 VDC source, capable of supplying a minimum of 50 ma. Power is supplied to the demo board through a barrel connector at the top right corner of the board. Any standard Microchip 9 VDC power supply will work, which has the appropriate mating connector. DS41407A-page Microchip Technology Inc.

29 System Detail A.4 SOFTWARE DESIGN The flowchart for the system software is shown in Figure A-8. FIGURE A-8: SYSTEM FLOWCHART Turn on Driver Select Reference Input Get ADC Select Sensor Input Get ADC Turn off Driver Get ADC Calculate Normalized Value Value < Threshold Yes No Average New Value Button Pressed Logic A timer interrupt driven function scanning is responsible for scanning and interpreting the impedance values from the buttons. The scanning function first selects a specific coil and turns on the PWM. It then measures both the combined impedance of the button and reference coils: MUX_ENABLE = ENABLE; RX_SEL = BUTTON_COIL; PWM_OUTPUT = PWM_ON; delay = STABILITY_TIME; while(delay--); // Enable the 4053 mux // Make sure to select button // Delay for stability // Delay Pad_Value = get_adc_value(); // Read ADC Next, it measures the impedance of just the reference coils: RX_SEL = REF_COIL; // Make sure to select button delay = STABILITY_TIME; // Delay for stability while(delay--); // Delay Ref_Value = get_adc_value(); // Read ADC 2009 Microchip Technology Inc. DS41407A-page 29

30 Inductive Touch Demo Board User s Guide Then, after turning off the PWM and isolating the detector, it determines a value for the virtual ground of the system: MUX_ENABLE = NOT_ENABLE; PWM_OUTPUT = PWM_OFF; delay = STABILITY_TIME; while(delay--); // Enable the 4053 mux // Delay for stability // Delay Vref_Value = get_adc_value(); // Read ADC Using this information, the function then generates a normalized value for the sensor coil: pressvalue = Pad_Value - Ref_Value; // Get just button value pressvalue <<= 10; // Multiply by 1024 pressvalue = pressvalue / (Ref_Value - Vref_Value);// Key pad reading It then compares the value against a trip threshold based on the coil s running average and the trip value specified for the coil: smallavg = averagedata[button]>>4; threshold = smallavg - (tripdata[button]>>4); if(pressvalue < threshold) { _KeyPressed = TRUE; } // Set flag for key pressed And, finally, it averages the new value into the running average if the button is not pressed: else if (pressvalue >= (threshold - 5)) { _KeyPressed = FALSE; // Set flag for no key pressed if (averagectr[button]++ > AVG_DELAY) { averagedata[button] -= smallavg; averagedata[button] += pressvalue; averagectr[button] = 0; } } DS41407A-page Microchip Technology Inc.

31 System Detail The actual conversion of a value out of the detector is performed by the get_adc_value(void) function. It performs 4 conversions of each value and adds the results together to obtain a 12-bit value from the 10 bit ADC. This increase in resolution is possible due to the random noise present at the output of the detector. WORD get_adc_value(void) { BYTE Tacq; BYTE i; WORD ADC_Val = 0; DWORD ADC_Avg = 0; while(adgo == 1); // Wait until done } for (i=0; i<4; i++) // Do 4 samples and average { Tacq = ACQ_TIME; // Wait the needed acq time while(tacq--); ADGO = 1; // Start ADC conversion while(adgo == 1); // Wait until done ADC_Val = (unsigned int)adresh<<8; // Read high byte value ADC_Val = (unsigned int)adresl; // Read high byte value ADC_Avg += ADC_Val; } return((word)adc_avg); // Return the average value The main line program then interprets the button press, lights the appropriate LED, and generates the appropriate tone. A.5 BOARD LAYOUT AND SCHEMATIC FIGURE A-9: INDUCTIVE TOUCH DEMO BOARD LAYOUT 2009 Microchip Technology Inc. DS41407A-page 31

32 Inductive Touch Demo Board User s Guide FIGURE A-10: INDUCTIVE TOUCH DEMO BOARD SCHEMATIC DS41407A-page Microchip Technology Inc.

33 INDUCTIVE TOUCH DEMO BOARD USER S GUIDE Index Numerics 5V power supply C Coil Selection Multiplexers Communication tab Customer Notification Service... 8 Customer Support... 9 D Demonstration Board...11, 13 Documentation Conventions... 6 Layout... 5 F Fascia Flash device H Hardware Design Current Driver Circuit Schematic of the Detector Section Simplified Measurement Circuitry I In-Circuit Debugger Inductive Touch Hardware Internet Address... 8 Introducing mtouch K Kit Components M Microchip Internet Web Site... 8 Microchip sales office MM74HC MM74HC MPLAB ICD MPLAB ICE mtouch Diagnostic Tool mtouch GUI P PICDEM 2 Plus Kit. See Kit Components. PICkit Serial Program source code R Reading, Recommended... 7 Readme... 7 S Sample code Sample Devices Sensor Design Settings menu Software Design Reference coils System Flow Chart Source files System Detail T Tool configuration menu Typical Sensor/Target Sandwich W Warranty Registration... 7 WWW Address Microchip Technology Inc. DS41407A-page 33

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